Apparatus for forming half-tone line screen with a lens



Oct. 20, 1970 e. K. STARKWEATHER 3,535,036

APPARATUS FOR FORMING HALF-TONE LINE SCREEN WITH LENS 5 Sheets-$heet 1 Filed March 6, 1968 INVENTOR. GARY K STARKWEATHER 1970 G. K. STARKWEATHER' 3,535, 5

TONE LINE SCREEN WITH A LENS APPARATUS, FOR FORMING HALF- 5 SheetsQSheet 2 Filed March '6, 1 968 INVENTOR.

GARY K. STARKWEATHER BY M7 AT TORNEVS G. K. STARKWEATHER 3,535,035-

APPARATUS FOR FORMING HALF-TONE LINE SCREEN WITH LENS 7 Filed March 8, 1968 I 3 Sheets-Sheet 5 INVENTOR. GARY K. STARKWEATHER ATTORNEYS Patented Oct. 20, 1970 US. Cl. 355-11 1 Claim ABSTRACT OF THE DISCLOSURE An apparatus for optically imaging a half-tone pattern on a xerographic photoreceptor that has been previously charged having an optical imaging means in the form of a multiple strip lens for imaging a line or dot pattern on the moving photoreceptor.

This invention relates in general to xerography and in particular to apparatus for permitting full development of large solid areas and continuous-tone images placed on the xerographic photoreceptors.

In xerography, a special xerographic photoreceptor comprising a layer of photoconductive insulating material placed upon a conductive backing is used to support xerographic images. The photoreceptor may be formed in any shape. An image is formed by uniformly electrostatically charging the photoreceptive surface and then exposing it to a radiation pattern in the form of the image to be reproduced. This radiation selectively discharges areas of the photoreceptor forming an electrostatic charge pattern conforming to the radiation image. This radiation image is generally an original document or other object which is illuminated and imaged on the photoreceptor.

The latent image on the photoconductive layer is then developed by contacting it With a finely divided electrostatically attractable material such as a resinous powder hereinafter called a toner. The toner is held to the image areas by electrostatic charge fields on the layer. The toner is held proportionately to the charge field so that the greatest amount of material is deposited where the greatest charge field is located. Where there is a minimum charge there is little or no material deposited. Therefore, a toner image is produced to conform with the latent image previously placed on the photoreceptor. In reusable xerographic systems the toner is transferred to a sheet of paper or other support surface and suitably fixed thereto to form a permanent print. This fixing may take p ace by heat or vapor which fuses the toner to the support material to which it has been transferred.

The toner used to develop the image is generally formed in a developer material consisting of the toner and a carrier of larger granular beads formed with glass, sand or steel cores coated with a material which is removed in the triboelectric series from the toner so that there is a triboelectric charge attraction between the toner and the carrier. The charge causes the toner to adhere to the carrier making it easily handled in a developer system which brings the developer into contact with the previously exposed xerographic surface. Because the charge pattern on the xerographic surface has a greater attraction for the toner than the triboelectric charge the toner has with the carrier the toner is then attacted electrostatically to the image charge pattern on the surface forming a visible tone image thereon.

Excellent results have been obtained in developing systems for a xerographic apparatus using cascade development with toner-carrier developer mixtures. This is especially true in continuous xerographic machines used for line copying of documents such as letters or other materials containing dark data on a light background.

Due to electric field conditions in the regions of the electrostatic images, however, large solid areas do not develop uniformly. Xerographic development of such areas delineate only their outline developing only in the areas where there is a differential in charge on the xerographic surface. In these cases the centers of these areas of high charge, being large solid areas of dark input, are not being developed or filled with the toner. Methods have been disclosed for overcoming the difiiculties encountered in developing solid area and continuous tone images with a cascade development system.

L. E. Walkup Pat. No. 2,598,732 discloses a method and apparatus for overcoming these difiiculties in stationary flat plate xerographic equipment by exposing the xerographic plate to a screen pattern in addition to the image pattern to be recorded, thereby breaking up the image into a halftone pattern. However, Walkup does not disclose means which are suitable for application to automatic machine where the surface carrying the electrostatic image is moving, such as in the case with a rotating xerographic drum or a web photoreceptor.

'C. F. Carlson Pat. No. 3,120,790 discloses apparatus for shadowing a half-tone pattern to a xerographic member by directly illuminating a rotating xerographic drum through a semi-transparent plate or screen containing opaque lines and a transparent background with the lines running parallel to the rotation of the xerographic member. This screen is positioned to extend over the entire exposure slit of the xerographic apparatus so that various portions of the image are always blocked by the opaque lines of the screen. The screen may be placed in a secondary location forward or aft of the exposure of the object to be copied by the machine. In any case the screen employed must include transparent and opaque lines running parallel to the axis of rotation of the xerographic member. The screen must be maintained closely spaced to the member for improved resolution during non-imaging exposure to the member by a shadowing technique.

Another method for achieving solid area and continuous-tone renditions in xerographic machines is described in the copending application of John M. Faw entitled Xerographic Exposure Apparatus, Ser. No. 645,193, filed on June 12, 1967. The apparatus used in the Faw application places an opaque sheet with lines thereon at the object plane of the exposure system of the xerographic machine. The lines are parallel to the rotation of the xerographic photoreceptor. The document or object is continually moved at the exposure station of the xerographic apparatus while the xerographic member rotates through its imaging stations optically viewing both the lined opaque screen and the object to be imaged both at the object plane of the exposure station of the xerographic apparatus. This motion causes the continual projection of the screen and the document onto the moving xerographic surface during the exposure process.

The screen extends across the entire width of the document and covers a portion of the object exposure slit being projected to the xerographic member at the exposure station of the xerographic apparatus. The screen has light and dark lines parallel to the direction of motion of the document and the xerographic member and therefore breaks up the charge pattern on the xerographic surface concurrently with the imaging of the document. The application describes the use of apparatus for providing a line screen pattern with a xerographic system in which the document remains stationary while the xerographic member moves by moving the line screen with the optical scanning elements traversing the document. The opaque screen employed must have lines thereon that are parallel to the motion of the photoreceptor.

Another method of achieving a half-tone image is disclosed in the copending application entitled Half Tone Method and Apparatus for Solid Area Coverage," Ser. No. 710,938, filed on Mar. 6, 1968 by G. Starkweather. There is disclosed a method and apparatus for achieving a half tone effect on the charge pattern of a moving xerographic member by periodically flashing a pattern on the moving member using shadowing techniques to affect a change on the member. No imaging of a pattern or slit occurs nor is contemplated in that application.

An inherent problem with the systems mentioned thus far which are capable of producing a half tone pattern on a moving photosensitive surface is one of resolution of the pattern on the surface. Because there is no imaging by these methods or apparatus disclosed, the pattern formed on the surface is a dispersing shadow of the slit or pattern held relatively close to the surface. Because of the sensitivity of the photosensitive surface it is impossible to maintain the pattern in a contact position with the surface. Without this, resolution is limited in direct relation to the spacing between the pattern and the surface. The present invention eliminates the problem of spacing of the pattern or any hardware near to the photosensitive surface to achieve good resolution since it images optically on the sensitive surface that pattern which is intended to be placed on the surface. The methods and apparatus disclosed herein permit sharp highly resolved patterns to be imaged on the photoreceptive surface forming distinct, sharp differentials in the charge on the previously uniformly charged photosensitive surface.

It is an object of this invention to improve the methods and apparatus for making copies of large solid area and continuous tone input on automatic xerographic machines.

Another object of this invention is to permit sharp halftone images on moving xerographic members.

Yet another object of this invention is to improve resolution capabilities of line screen and another half-tone pattern used with moving xerographic members.

These and other objects of this invention are achieved by exposing a moving xerographic member to a halftone pattern through an imaging member such as cylindrical lens or a multiple strip lens capable of imaging an object precisely on the xerographic member which moves in the image plane of the lens. For a better understanding of the invention as well as other objects and further features thereof, reference is had to the following detailed description of the invention to be read in connection with the accompanying drawings wherein:

FIG. 1 is a schematic representation of a half-toning apparatus embodied in xerographic structure;

FIG. 2 is a schematic representation of another embodiment of half-toning apparatus;

FIGS. 3-5 are isometric views of a half-toning apparatus;

FIG. 6 shows a schematic electrical diagram of a circuit for achieving the required illumination of the apparatus of FIGS. 4 and 5.

There is shown in the figures a xerographic copying apparatus comprising a xerographic member or photoreceptor formed in the shape of a drum 20 in FIG. 1 or a belt 22 in FIG. 2. The drum of FIG. 1 is mounted on a shaft 24 suitably journaled to rotate in the direction indicated by the arrow to cause the drum surface to sequentially pass a plurality of xerographic processing stations. For the purpose of the present disclosure, the several xerographic processing stations in the path of movement of the xerographic member may be described functionally as follows:

A charging station A at which a uniform electrostatic charge is deposited on the photoconductive layer of the drum 20;

An exposure station located preferably as shown by reference character B at which a light or radiation pattern of copy is exposed onto the xerographic photoreceptive surface to dissipate the drum charge in the exposed portions thereof forming a latent electrostatic image of the pattern of the object 26 to be copied;

Adjacent the exposure station is an exposure mechanism generally designated 30 to prepare the charge pattern on the drum for development of solid area and continuous-tone images;

A developing station C whereat the latent electrostatic image is developed by cascading an electrostatic developer over the drum forming a toner image corresponding to the latent electrostatic image on the drum;

An image transfer station D where the toner image is electrostatically transferred from the drum surface to a transfer material; and a cleaning station E where the drum is cleaned of residual toner and is discharged in order to prepare the drum surface for the next cycle.

The transfer material then passes through a fusing station F where heat is applied to the material in a sufficient quantity to melt the electrostatic toner image thereon forming a bond between the toner and the transfer material to which it is adhered.

In FIG. 1 the xerographic drum is rotated about its shaft 24 by suitable drive means (not shown). While it is rotating, a scanning system operates at the exposure station through lamps 32 and lens 34 presenting an image of the object 26 at the xerographic drum under exposure slit 36. For a more detailed description of scanning apparatus such as the type that is schematically shown see Rutkus Pat. No. 3,062,095 issued Nov. 6, 1962. Positioned near the exposure slit 36 is the exposure mechanism for providing the charged surface of drum 20 with capabilities of development of solid area and continuous-tone image formations on the drum. The exposure mechanism shown may be positioned along the drum surface anywhere between the charging station A and the developer station C.

The exposure mechanism 30 is comprised of a light baffle 38 having a slit 40 therein extending the entire length of the drum 20 transverse to its rotation. Enclosed within the exposure mechanism is a lamp 41 of such construction as to extend transverse across a substantial portion of the drum surface 20. The lamp may be of a fluorescent, incandescent, xenon, mercury or other gaseous filled type. The lamp is electrically connected to a suitable electric circuit.

The exposure mechanism 30 is comprised of an imaging system for imaging an object presented at the plane 42 onto the surface 20 of the moving light sensitive drum. The imaging means is a strip lens device 44 which presents a 1:1 magnification of the object at plane 42 to the image plane or light sensitive drum 20. A more complete description of a lens strip will accompany the description of FIG. 3.

The object presented at object plane 42 is shown here to be a continuous web 45 looped around a drive roller 46 and two guide rollers 47 and 48. The drive roller 46 drives the object web 45 between guide rollers 47 and 48 at a surface speed substantially equal the surface speed of the drum 20 passing beneath the slit 40 in the light baffle 38. The object web 45 contains a half-tone pattern for imaging onto the drum 20 discharging it in a manner to permit full development of solid areas and continuous tone images that are presented from the object 26.

A variety of patterns may be used on the object web 45. Transparent areas may comprise rows of small squares entirely surrounded by black or opaque areas. A checkerboard pattern of transparent and opaque areas may be used. The transparent area may be continuously covered with circular black or opaque dots. It may be transparent dots or squares or diamonds or other similar designs on a black or opaque background and there may be alternate rulings of transparent and opaque lines in any orientation to the rotation of the web 45 and the drum 20. Any of several sizes of alternate rulings or designs may be used in the pattern. A coarse pattern having 50 or dots or lines to the linear inch could be useful for some purposes.

Finer patterns having 100 to 40-0 and even more dots or lines to the inch will give more nearly a continuous tone appearance to finished copy or prints.

The guide rollers 47 and 48 maintain the web 45 in a more or less flat orientation substantially within the ob-,

ject plane 42 of the strip lens 44. By illuminating through the transparent portions of the half tone pattern web 45 with a lamp 41 a flowing image of the pattern is produced through the lens 44 to the drum 20 at the slit 40 in the light baffle 38.

FIG. 2 shows a xerographic apparatus similar to that in FIG. 1. The xerographic belt 22 is moved by drive roller 50 and around idler rollers 51 and 52. The belt is exposed along a flat portion at station B by flashing high intensity lamps such as xenon flash tubes at a platen 54 containing an opaque object 56 causing a reflection creat- 1ing a radiation pattern through lens 53 for exposing the alt 22.

At a portion of the belt between the charging station A and the developing station C is positioned an exposure mechanism to present a finely divided, uniformly distributed pattern of light to the belt prior to developing. The mechanism here operates by having a half-tone pattern 60 having dark areas on a reflective or white opaque background lighted by lamps 62 to cause a reflection of the pattern 60 through lens 63 to the moving photoreceptor 22 passing at an image plane of lens 63. Lens 63 may by spherical or aspherical but should be capable of imaging in more than one meridian. It is preferably a spherical lens. The pattern 60 is positioned at the object plane of the lens 63. The pattern 60 may be composed of lines running in a direction parallel to the movement of the surface 22 at the image plane of the lens 63 where a latent image of the pattern is formed by selectively dissipating the electrostatic charge on the belt 22 in the areas of light reflection imaged onto the belt through the lens 63 from the object pattern 60. Imaging can employ a scanning mechanism as in FIG. 1 or the type shown in Rutkus patent supra where the object remains stationary eliminating restrictions on image orientation. If the lamps are connected to an electrical circuit such as is shown in FIG. 6 a similar broadening of pattern restrictions can be achieved.

FIG. 3 shows a schematic representation of a strip lens device used in conjunction with a xerographic drum. A xerographic drum 20 is rotated in a direction shown by the arrow through the image plane of the lens strip system. At the object plane of the strip lens is a half-tone pattern 70 containing opaque lines 72 on a transparent background. The lines 72 are parallel to the direction of rotation of the xerographic drum 20. A lamp 77 is positioned on the side of the pattern 70 opposite from the strip lens to provide a light source for imaging the pattern 70 through the strip lens 44 to the image plane on the xerographic drum 20. The lamp contains radiation at frequencies to which the photosensitive surface of the xerographic drum 20 is sensitive so that discharge of the surface occurs in the areas where the light is imaged.

A description of the operation of a strip lens is found in copending application Ser. No. 683,987 filed in the names of R. W. Gundlach et al. on November 17, 1967 as a continuation of application Ser. No. 568,925 filed on July 29, 1966 and titled Improvements in Lens Strip Optical Scaning System. Briefly, the lens strip permits a short conjugate optical system to cover a large area in at least one plane or a large format in two planes if the optical system is of the scaning type. A plurality of optical imaging devices 73 make up strip 44. Each device 73 has a plurality of lens elements 74, 76, 78. The first set of lens elements 74 forms a virtual image of the object at the object plane onto the second set of lens elements 76. The third lens element 78 images from the second onto the image plane. Suitable field stops 81 are placed near the second strip lens elements 76 so that the images formed by each plurality of optical imaging devices 73 touches or overlaps at the image plane with the images of other devices 73 giving a uniformly illuminated continuous image across the image plane of the particular object being imaged. An aperture stop is included to provide better images than would otherwise be obtainable.

Each of the plurality of optical imaging devices 73 in the system 44 is adapted to reproduce an elemental area of the object plane containing the half-tone pattern, for example, the pattern on plate 70 and form corresponding images on the image plane here being the xerographic surface 20. The composite of the elemental images so produced will represent the complete pictorial representation of the information on the object plane plate 70. Lamp 77 illuminates the plate 70 for imaging as lines of light 79 on the photosensitive surface 20. Field stops 81, light baffles 83 and aperture stops 85 are extended across all of the plurality of imaging devices 73. The aperture stops are oblong or rectangular opening with the long dimension parallel to the direction of the light lines 72. This maximizes light efiiciency while mimimizing aberration effects from portions of the lens not required to image.

Another lens system for imaging a half-tone pattern onto a moving light sensitive xerographic surface is shown in FIG. 4. Here a point light source 80 is imaged through a cylindrical lens 82 as a line 84 on the image plane of the lens through which drum 20 passes. The cylindrical lens is formed of portions of cylinders rather than spheres which form lenses such as the individual components of the strip lens of FIG. 3. The cylindrical lens has no refractive power in one meridian with a maximum power in another from the first. Therefore, with the lens positioned as shown the dot or point source of light imaged through the cylindrical lens will not be dispersed or expanded in a direction parallel to the direction of rotation of the photosensitive surface 20 in FIG. 4. The point source will be, however, dispersed along the axis perpendicular to the direction of rotation of the surface 20. Because the cylindrical lens images the point source as a line at its image plane the line of light is sharp and distinct with a high resolution unlike the results achieved by shadowing techniques.

By electrically connecting the point source 80 to an electrical circuit 87 such as that shown in FIG. 6 and pulsing the light source, a periodic imaging of a line on the drum occurs. By adjusting the rate of pulsing of the light source the lines may be imaged relatively close to each other on the surface resulting in areas of greater and lesser charge density for good large solid area and continuous tone rendition of copy from the xerographic apparatus in which this exposure mechanism is embodied.

As the photoreceptive surface rotates through the image plane of the cylindrical lens 82 the lamp 80 is pulsed by an appropriate circuit, such as that shown in FIG. 6. Assuming that the surface speed of the photoreceptive surface 20 is approximately 10 per second and that point source of light is approximately one milimeter in diameter, the image through the cylindrical lens onto photoreceptive surface at unit magnification will be a line one milimeter in width and extending across the entire length of the drum surface 20. The lamp, when ignited, will expose the photoreceptive surface to the image of the light source 80. This image is the line 84. If the lamp is ignited and turned off x times per second, a halftone line screen having x light lines with a corresponding number of dark spaces in between per linear measurement of drum circumference. If the drum rotates at 10" per second there will be x/ 10 line images of charge differential per inch. It has been discovered that a half-tone screen comprising approximately dark region and 5% exposed or light region on the photoreceptive surface renders excellent results in developed images oflarge solid area and continuous tone copy. Therefore, a pulsing of the light source 80 should provide illumination for a minimum of about 5% of the total drum length between the beginning of a line image of light 84 on the drum and the beginning of the next line image having a dark space between the two line images.

FIG. shows a modification of the cylindrical lens type mechanism of FIG. 4. Here we have several light sources 86, a, b and c placed in the object plane of the cylindrical lens 82. Each point source is imaged as a line in the image plane of the cylindrical lens 82 at the conjugate point corresponding to its position at the object plane. Therefore, with the three point sources shown, three distinct parallel lines will be imaged at the image plane of the cylindrical lens through which passes the xerographic drum 20 as it rotates in a xerographic apparatus in which it is maintained. By providing for more than one ight source, lamps of lesser intensity are used to expose the xerographic drum for half-toning application. This gives greater lamp life and lesser power requirements for the circuit and machine embodying the exposure mechanism. By pulsing the lamps at a rate similar to that which would be used for FIG. 4 and having each lamp at approximately /3 the intensity of the lamp required for FIG. 4 the differences in charge density on the xerographic drum would be the same in both cases provided each image area of the lamp sources of FIG. 5 was struck by images of each of the three point sources; This can be done by strategic placing of point sources in the object plane of the cylindrical lens 82 in a relation to the speed of rotation of the xerographic drum 20 through the image plaen of the cylindrical lens 82. The number of light sources is not limited to three nor one nor any particular number but can be increased or decreased as the radiometric or mechanical limitations of the system require.

FIG. 6 shows one embdoiment that may be used to cause a periodic flashing of a lamp such as the lamp 80 in FIG. 4. In the figure there is shown a lamp enclosed 1n an envelope 90 containing a material capable of transmitting desired wave lengths of radiation at a high intensity to discharge the photoconductive surfaces of the xerographic members 20 or 22. Lead-in wires 91 are embedded in the envelope, each lead-in wire bearing an electrode maintained in a spaced apart relationship.

The anode electrode 92 is connected by an electrical conductor 94 to the positive terminal of a capacitor 96 that may be charged through a charging resistance 97 from an energy source such as battery 98 when switch 99 is closed. The other or cathode electrode 93 is connected to the negative terminal of capacitor 96. In combination with the lamp charging circuit is a trigger circuit 102 including, for example, a radio frequency source 103 and an external winding 104.

The trigger circuit provides for an alternative manner to ignite the lamp. For example, capacitor 96 may be charged, as previously described, to a voltage below the breakdown voltage of the particular conditions of the flash lamp 101. The lamp may then be triggered by means of a trigger circuit 102 transmitting an impulse from radio frequency source 103 to the external winding 104 to cause a partial ionization of the gaseous medium Within the envelope 90 making the medium conductive enough to permit the voltage stored in the capacitor 96 to become discharged through the gaseous medium, from the anode 92 to the cathode 93 thereby producing a high intensity radiation in the wave length regions required.

Many other trigger devices are suitable for use herein. For example a, charged silver strip painted on part of the. envelope or charged metallic reector can also serve as an external trigger electrode causing the lamp to fire. The lamp itself may be filled with xenon, mercury vapor, or any other gases which would present the desired radiometric results for discharging the xerographic plate. There are presently available xenon lamps having high frequency, low energy flashes that have a lamp life of 10 flashes to 50% of output providing 600 pulses per second at .008 joules per pulse. This and other lamps may be used with the apparatus schematically described in this application for achieving the results stated herein.

While the invention has been described with reference to the structure disclosed herein, it is not confined to the details set forth, and this application is intended to cover such modifications or changes as may come within the purpose of the improvements or the scope of the following claim. What is claimed is: 1. An exposure device for use in a xerographic reproducing apparatus comprising a moving photoconductive surface charging means to place a substantially uniform electrostatic charge on the surface first imaging means to form a latent electrostatic image on the charged photoconductive surface, second imaging means to optically present a light pattern capable of giving finely divided, uniformly distributed charge differentials to the moving xerographic photoconductive surface at portions capable of being simultaneously occupied by the image presented from the first imaging means, means to develop the latent electrostatic image, said second imaging means comprising a half-tone pattern and a plurality of optical imaging devices for projecting an image of the half-tone pattern from an object plane to an image plane of the plurality of optical imaging devices, the moving photoconductive surface passing through the image plane and the half-tone pattern in a corresponding object plane of said plurality of optical imaging devices, each of said devices having a plurality of lens elements arranged along a common optical path extending between the object plane and the image plane. said half-tone pattern comprising a continuous web and has further associated therewith a drive means to move the web through the object plane of the plurality of optical imaging devices in synchronous speed with the movement of the photoconductive surface passing through the image plane of the plurality of optical imaging devices so that the pattern and sur face are optically fixed relative to each other.

References Cited UNITED STATES PATENTS NORTON ANSHER, Primary Examiner D. J. CLEMENT, Assistant Examiner US. Cl. X.R. 355-3, 8, 16 

